Lithium niobate thin films and lithium tantalate thin films are widely used in optical signal processing, information storage, electronic devices, etc. For example, lithium niobate thin films and lithium tantalate thin films can become substrate materials for making opto-electronic devices and integrated optical circuits with high frequency, high bandwidth, high integration, high capacity and low power consumption.
In 1992, Bruel raised a method to peel thin films from semiconductor bulk materials, which is called smart cut, whose procedures mainly comprises technologies of ion implantation, bonding, thermal splitting and surface polishing of thin films. Specifically it starts with implantation of gas ions such as hydrogen ions or helium ions in the original substrate. The quantity distribution of the implanted ions at different depths is presented as Gaussian distribution. Most of the implanted ions stay at a certain depth under the surface of the original substrate. Said certain depths are decided by the energy of implanted ions. The area where the implanted ions concentrate and stay is called the splitting layer, whose thickness ranges from tens of to hundreds of nanometers or so. The part of the original substrate between the splitting layer and the upper surface of the original substrate is the thin film layer which is to be peeled off. The other part is the remaining material layer to be left after the peeling. The thin film layer and the remaining material layer are substantially free of implanted ions. Make the target substrate face and become in contact with the thin film layer of the original substrate, so as to carry out the direct bonding of wafers and form a bonding unit. The bonding unit is then heated. The heating makes the temperature rise over 450□ (namely, splitting temperature), making the thin film layer split from the remaining material layer. The heating is of two functions. The first function is to enable the implanted ions in the splitting layer to obtain ample energy to break away from the bondage of substrate ions so as to make break the bond formed between the implanted ions and the substrate ions, the implanted ion turning into gas atom (for example, the implanted helium ions become He), or meeting with another detached implanted ion and turning into gas molecules (for example, one implanted hydrogen ion meets with another hydrogen ion and become H2). The gas atoms or molecules occupy a certain volume and sequentially some tiny cracks are formed in the splitting layer. As the heating time increases or heating temperature rises, the gas atoms or molecules accumulate and get aggregated with each other, thus giving birth to tiny bubbles in the splitting layer. Then the bubbles expand into a whole, finally making the splitting layer breaks, and sequentially separating the thin film layer which is bonded to the target substrate from the remaining material layer. The other function of heating is to strengthen the bonding force between the bonded wafers (namely, strengthen the bonding force between the original substrate and the target substrate). After the splitting of the remaining material layer from the thin film layer and the target substrate, anneal the thin film layer and the target substrate at a high temperature (generally, the temperature is above 600□; wherein, the target substrate is taken as the handle substrate of the thin film layer), so as to further strengthen the bonding force and exterminate the lattice defects formed in the thin film layer during the process of ion-implantation. Finally, polish the surface and get the thin film.
In this method, a very critical step is to split the bonding unit by heating. If this process is properly controlled, then intact thin films with few defects can be obtained. In spite of the above-mentioned method of splitting by heating, there are still some other methods to split the bonding wafers, such as high-pressure water gun splitting method, microwave method, the combination of microwave and heating methods, etc. Therefore, different methods of fabricating thin films come into being. And these methods are widely used in the fabrication of silicon on insulator (SOI).
However, when methods similar to the smart cut method are used to form thin films whose splitting temperature is low (for example, oxide crystal materials thin film such as lithium niobate thin films or lithium tantalate thin films, GaAs thin films, etc.), there will be bubbles appearing on the thin film at last, which results in a very poor quality of the finished thin film products.